Oceanographic probes generally consist of a deployable probe unit which includes sensors for measuring the target characteristics, e.g., a conductivity cell for measuring electrical characteristics, and/or a temperature sensor for measuring thermal characteristics. Wires or other means are used for transmitting signals generated by the sensors to a shipboard station. The shipboard station includes instrumentation for translating the signals received from the sensors into data reflecting the characteristics being measured.
Existing probe technology typically makes use of two types of systems to acquire seawater property data as a function of depth. One type of system employs recoverable sensors that can be lowered by cable from a ship or suspended from a moored or free-drifting buoy. The sensors used on such recoverable systems are very accurate, but are typically large and expensive to purchase, maintain, and operate. The depth accuracy of the recoverable sensor system is limited only by the accuracy of the pressure sensor that is attached to the probe. Typical pressure sensor accuracy for a recoverable sensor is .+-.0.5%. In operation, the recoverable pressure sensor is monitored at every depth at which seawater property data is sampled to provide a depth measurement for the seawater property data collected. Other than high cost, another drawback of recoverable sensors is that the ship or buoys deploying them must be held nearly stationary while collecting data which prevents the collection of synoptic data over any reasonable distance.
The second type of seawater probe system employs expendable, single use, sensor probes that are dropped over the side of a stationary or moving ship or that are launched from an aircraft. Probe sensor data is transmitted to the ship over a wire link that deploys simultaneously from the ship and the probe as the probe free-falls through the water. A considerable quantity of data can be collected over a large area at a relatively low cost. However, the sensors on expendable systems are typically not as accurate as recoverable sensors. Because it is usually desirable to be able to reliably compare data generated by different probes, it is necessary to insure that different probes generate the same data under the same circumstances.
The depth of a probe is an important parameter to consider when taking almost any seawater property measurement. The depth of an expendable probe is inferred from the elapsed time at which each seawater property measurement is taken relative to the probe hitting the seawater surface, assuming a known descent rate. After water entry, it is assumed that the probe follows an empirically determined descent rate. One way to achieve repeatability in the descent rate among probes is to manufacture the probes to very exact weight and dimensions tolerances. The resulting usable depth accuracy of an expendable probe is limited to about .+-.2%, or .+-.15 feet, whichever is larger. Descent rate can also vary due to variations in seawater density and variations in initial probe velocity when it enters the sea surface. Although the accuracy of the various expendable seawater property sensors has improved, the relatively poor depth accuracy of expendable sensor systems has limited their overall usefulness.
Accordingly, it is an object of the present invention to provide an inexpensive, expendable sensor with depth accuracy comparable to typical recoverable systems.